Method and device for production of a quartz glass blank

ABSTRACT

A conventional method for the production of a quartz glass blank comprises a method step in which SiO 2  particles are generated by means of a series of deposition burners and deposited on a cylinder outer surface of a support, rotating about the longitudinal axis thereof to form a cylindrical porous SiO 2  soot body. The surface temperature of the forming soot body is altered by means of a temperature adjustment body. According to the invention, the above may be developed to give an economical method for the production of an SiO 2  soot body with low axial thickness variations and to provide a device of simple construction of the same, whereby the temperature adjustment body is applied in the form of a planar element running along a significant part of the SiO 2  soot body, which either acts on the soot body surface as a temperature-screening homogeneous heat sink or as a homogeneous reflector for temperature raising, by means of heat radiation. A device suitable for carrying out the above method is characterised in comprising a temperature adjustment body ( 13 ), with a planar element acting as a homogeneous heat sink or a homogeneous reflector which runs along a significant part of the SiO 2  soot body ( 2 ) and which has a given reflectance for IR radiation.

The present invention relates to a method for producing a quartz glassblank, the method comprising a step in which SiO₂ particles are producedby means of a row of deposition burners and deposited on a cylinderouter surface of a carrier rotating about the longitudinal axis thereofto form a cylindrical porous SiO₂ soot body, the surface temperature ofthe forming soot body being altered by means of a temperature adjustmentbody.

Furthermore, the present invention relates to a device for producing aquartz glass blank, comprising a row of deposition burners for producingSiO₂ particles, a carrier which is rotatable about the longitudinal axisthereof and on the cylinder outer surface of which the produced SiO₂particles are deposited to form a cylindrical porous SiO₂ soot body,comprising at least one temperature adjustment body which is arranged inthe area of the forming soot body and which acts on the surfacetemperature of the soot body for altering the axial density profilethereof.

Quartz glass blanks are used in the form of tubes or rods, especially assemi-products for producing optical components and optical fibers. Theaxial and radial optical homogeneity of the quartz glass blanks is herea decisive quality criterion. The blanks are obtained by sinteringcylindrical porous SiO₂ preforms (“soot bodies”) which are formed bylayerwise deposition of SiO₂ particles on a rotating deposition surfaceby means of a plurality of deposition burners. Only soot bodies with auniform particle distribution and a narrow density band over their wholelongitudinal axis can be processed into high-quality quartz glassblanks.

A method and a device according to the above-mentioned type are knownfrom DE-C 198 27 945. The production of an elongate porous soot body ofSiO₂ particles is described therein, wherein SiO₂ particles aredeposited in layers by means of flame hydrolysis burners onto ahorizontally oriented carrier rod which is rotating about itslongitudinal axis. The burners are mounted at an equal distance relativeto one another on a burner block extending in parallel with thelongitudinal axis of the carrier. The burner block is reciprocated alongthe forming porous cylindrical soot body between left and rightturnaround points by means of a controllable displacement means, theamplitude of said translational movement being smaller than the sootbody length. In the area of the turnaround points the soot body surfaceis overheated, resulting in local axial density variations. To avoidsaid axial density inhomogeneities, it is suggested in DE-C 198 27 945that the soot body surface should be cooled actively or passively in thearea of the turnaround points. In the case of active cooling, heat isdischarged from the soot body surface in the area of the burnerturnaround points, e.g. by means of cooling elements or by heatconvection or heat flow. In the case of passive cooling, heat sinks areprovided in the area of the turnaround points, and these are configuredas absorbing surface areas or as slits in a heat shield surrounding thesoot body.

Thanks to the heat shield a heat loss in the areas between theturnaround points is reduced and it is promoted in the area of theturnaround points. Hence, these cooling measures have atemperature-reducing effect locally restricted to the areas of therespective turnaround points.

A further method for avoiding temperature peaks in the area of theturnaround points is suggested in DE-A 196 28 958. An overheating of thesoot body is here prevented or reduced in the areas around theturnaround points in that the rotational velocity of the forming sootbody is increased in said areas, the flame temperature of the depositionburners is reduced, or the distance of the deposition burners from thesoot body surface is increased. With these measures an increase intemperature in the area of the turnaround points can be compensated inpart or fully and axial density gradients in the soot body can beavoided or reduced.

The known methods have in common that for compensating or avoiding axialdensity differences high constructional or controlling efforts have tobe taken and that the suggested compensating measures are limited to thearea of the turnaround points of the burner movement.

However, due to different burner characteristics, due to differences inthe burner adjustment or due to misalignments as a result of temperaturevariations during the deposition process, irregular temperature effectson the soot body are bound to be observed also outside the turnaroundpoints of the burner movement, and thus inhomogeneous density profilesover the longitudinal axis of the porous SiO₂ soot body. Such densityvariations make it difficult to uphold predetermined quality standardsof the quartz glass blank.

As a rule, the deposition process takes place in a deposition chamberwithin which the row of burners and the soot body as well as thenecessary mounting components and lines are arranged, and which isfrequently provided with an inspection window. Therefore, due to leakageradiation on differently reflecting surfaces inside the depositionchamber, temperature differences will be observed in the area of thesoot body surface also in cases where identical properties of thedeposition burners of the row of burners are present, which constitutesa precondition that could hardly be met even if the deposition burnerswere replaced by a single slit burner extending along the soot bodysurface.

It is therefore the object of the present invention to provide aninexpensive method for producing an SiO₂ soot body with little axialdensity variations and to provide a constructionally simple devicetherefor.

As for the method, this object starting from the method of theabove-indicated type is achieved according to the invention in that thetemperature adjustment body is used in the form of a planar elementextending along a substantial part of the SiO₂ soot body, which eitheras a homogeneous heat sink has a temperature-shielding effect on thesoot body surface or, as a homogeneous reflector, a temperature-raisingeffect due to heat radiation.

The following formula is in general applicable to the impingement ofelectromagnetic radiation (light) on a surface:R+S+A+T=1where R=reflectance, S=degree of leakage, A=degree of absorption, andT=degree of transmission. In the case of mirror-reflected light, theangle of incidence=angle of emergence, whereas in the case ofdiffuse-reflected light, the angle of emergence is no longer relatedwith the angle of the incident light.

In the method of the invention, the temperature adjustment body has aplanar element which acts either as a homogeneous heat sink or as ahomogeneous reflector.

The difference with respect to the known method is that with the planarelement it is not the surface temperature of individual discreteportions of the forming soot body that is lowered, but the element actson the surface temperature over the whole usable length of the body in ahomogenizing manner. This effect is achieved in that the planar elementis designed as a homogeneous temperature-shielding heat sink or as atemperature-raising homogeneous reflector. In the case of aconfiguration of the planar element as a reflector, a temperatureincrease over the whole soot body surface is aimed at by predeterminingthe reflectance for the IR radiation. This has the consequence thatlocal temperature peaks are evened out, i.e. independently of whethersaid temperature peaks are created by the burner movement or whetherthey are due to misalignments or differences between the individualdeposition burners or due to leakage radiation.

If the planar element is configured as a heat sink, local temperatureincreases due to leakage radiation are prevented or avoided in that theleakage radiation is absorbed or dissipated. Hence, this procedure hasalso the consequence that local temperature peaks are avoided.

To enable the planar element to develop one of these effects, it isconfigured either as a mirror element (reflector) which homogeneouslyreflects IR radiation, or as a cooling body (heat sink) whichhomogeneously absorbs IR radiation. In the first-mentioned case thesurface design of the planar element is of essential importance whereasin the second case the material of the planar element additionallyinfluences the cooling function.

The planar element extends over a considerable part of the length of theforming soot body, and its temperature-homogenizing function isfulfilled all the more easily and better the longer the length sectionof the soot body is that is covered by the planar element. A planarelement which is slightly shorter than the soot body can still developthis homogenizing function to an adequate degree over the whole usablesoot body length. Therefore, for reasons of clarity, a partial length ofmore than 50% of the soot body length is still defined as a “substantialpart” of said length.

Of importance is the selective adjustment of the reflectance of theplanar element with the aim to even out the profile of the surfacetemperature and thus to homogenize the axial density profile of the sootbody. This adjustment of the effect of the planar element by surface ormaterial properties is carried out once at the beginning of a depositionprocess and will normally also be maintained in the subsequentdeposition processes.

In the method of the invention one planar element or several planarelements of equal effect may be used at the same time. It is alsopossible to use a plurality of planar elements that differ in theirhomogenization effect with respect to intensity or type (acting as ahomogeneous heat sink or as a homogeneous reflector), but it is ensuredat any rate that a planar element within the meaning of this inventionis used that extends along a substantial part of the SiO₂ soot body. Forexample, to achieve a lower surface temperature in the area of the endsof the SiO₂ soot body, planar elements may be provided with a differenteffect than that of the planar element acting on the central area of theSiO₂ soot body within the meaning of the invention.

Preferably, a planar element is used that is formed by an inner wall ofa housing surrounding the SiO₂ soot body.

This variant of the method is particularly simple in constructionalterms because the SiO₂ soot body is normally deposited in a depositionchamber. In this instance the planar element is integrated into the wallof the deposition chamber, so that it forms the wall itself or part ofthe wall. In the simplest case the whole inner wall of the housing formsa planar element within the meaning of the invention. It is alsoimportant here that the material and surface properties of the wall areset with respect to the functionality to be achieved, namely having atemperature compensating effect over the length of the soot body.

In a first preferred configuration of the method of the invention, theplanar element acts as a reflector with a reflectance for IR radiationbetween 80% and 100%.

It has been found that variations in the surface temperature areparticularly efficiently evened out by a planar element reflecting theIR radiation. The surface temperature of the soot body is here raised bymeans of the reflector to an altogether higher temperature level, withthe consequence that the amount of heat to be applied by the depositionburners can be lowered. It is thereby possible to increase thealtogether more homogeneous heating of the soot body surface by theinventive planar element at the expense of the more inhomogeneousheating by the deposition burners. Hence, the temperature profile ishomogenized on the whole over the length of the soot body. In thisconfiguration of the method, two variants have again turned out to beadvantageous.

In the first variant of the method, heat of the deposition burners isreflected by means of the planar element towards the soot body. Theplanar element is here arranged and configured such that heat emanatingfrom the deposition burners, which are arranged in a row, impinges onthe element and said heat is reflected towards the forming SiO₂ sootbody. The planar element may e.g. be arranged such that the row of thedeposition burners or the rows of the deposition burners extend betweenthe soot body and the planar element. The lost heat emitted by thedeposition burners to the rear is thus intercepted by the planar elementand directed towards the forming soot body.

In the second variant of the method, the heat of the forming SiO₂ sootbody is reflected by means of the planar element towards the soot body.

The heat emitted by the soot body is here intercepted by the planarelement and reflected back again towards the soot body. The planarelement preferably extends above, next to, or below the soot body inthis instance. The flame temperature of the deposition burners is higherthan the surface temperature of the soot body. Since the intensity ofthe temperature radiation is increasing approximately in proportion withthe fourth power of the temperature T (in degree Kelvin), a reflectionof the flame temperature has a stronger temperature-increasing effect onthe soot body than in the variant of the method where the heat emissionof the soot body is again reflected back to the body itself.

In a planar element acting as a homogeneous reflector, the temperatureprofile along the soot body surface is evened out in that part of theheat to be applied on the whole is increased by a more homogeneousheating manner (reflector) at the expense of a rather more inhomogeneousheating manner (deposition burner).

Advantageously, a planar element is here used which has an efficiency,defined as the solid angle covering the forming SiO₂ soot body, of atleast 60%.

As an alternative, a procedure has also turned out to be useful in whichthe planar element acts as a heat sink absorbing IR radiation.

In this variant of the method, the planar element does not have aheating or cooling effect on the soot body surface, but it just preventsor reduces the effect of the basically rather inhomogeneous leakageradiation on the soot body, so that the temperature profile is alsoevened out.

This effect as a heat sink is also achieved in a preferred variant ofthe method in which a planar element is used that has a roughenedsurface with a mean surface roughness R_(a) of at least 10 μm. Due tothe roughening of the surface the degree of leakage S is considerablyincreased. Hence, this procedure increases the amount of diffusereflection at the expense of the mirror reflection. In addition, heatradiation is eliminated by the specific absorption of the correspondingmaterial.

Such a roughened surface can be adjusted in a particularly simple andinexpensive way by grinding, freezing (etching), blasting or similarsurface treatment methods. The mean surface roughness R_(a) is heredetermined according to DIN 4768.

An equally temperature-homogenizing effect is achieved when use is madeof a planar element having a blackened surface.

The absorption degree A is considerably raised by blackening thesurface. This procedure reduces or eliminates, in particular, the effectof inhomogeneous leakage radiation, as may e.g. emanate from reflectingsurfaces inside a process chamber. The blackening may be provided inaddition or as an alternative to a roughened surface. Furthermore, aplanar element which acts as a heat sink has turned out to be usefulwhen it is cooled.

Cooling is achieved in that the planar element is brought into contactwith a coolant. The coolant may be a cooling gas, a cooling liquid or acooling body. This variant of the method has the advantage that thetemperature and thus the efficiency of the planar element can be variedby means of the coolant for influencing and homogenizing the surfacetemperature of the soot body within certain limits. The cooling of theplanar element may be provided in addition or as an alternative to aroughened surface and/or blackening.

Furthermore, it has turned out to be advantageous when the distancebetween the planar element and the surface of the forming SiO₂ soot bodyis kept constant.

This ensures a substantially constant temperature-homogenizing effect ofthe planar element during the deposition process. The planar element ise.g. shifted with an increasing diameter of the forming SiO₂ soot bodyin a direction perpendicular to the longitudinal axis of the carrier.

It has also turned out to be particularly useful to move the planarelement along the soot body.

This procedure is particularly of advantage in a planar element thatextends only over a partial length of the soot body. Moreover, thisyields a simplified construction in those cases where a fixed planarelement might impede the movement of the row of burners; for instance inan arrangement in which the row of burners extends between soot body andplanar element, so that the supply lines of the burner row would have tobe guided either through the planar element or extend thereabove. Themovement of the planar element can e.g. take place in synchronism withthe movement of the deposition burners along the soot body.

In a particularly preferred configuration of the method of theinvention, the planar element extends over the whole usable length ofthe soot body. This configuration of the planar element facilitates theadjustment of a homogeneous temperature distribution. The planar elementextends over the usable length or beyond said length. The usable sootbody length corresponds to the cylindrical length section of the sootbody without tapering portions at the two ends (end caps).

As for the device, the above-mentioned object starting from a device ofthe above type is achieved according to the invention in that thetemperature adjustment body comprises a planar element which acts as ahomogeneous heat sink or as a homogeneous reflector and which extendsalong a substantial part of the SiO₂ soot body and has a predeterminedreflectance for IR radiation.

In the device of the invention, the temperature adjustment bodycomprises a planar element that acts either as a homogeneous heat sinkin a temperature-shielding manner or as a homogeneous reflector in atemperature-raising manner due to heat radiation on the soot bodysurface.

The planar element extends at least over a partial length of the formingSiO₂ soot body. In contrast to the known device, the planar element isconfigured as a homogeneous heat sink or as a homogeneous reflector witha given reflectance. When the planar element is designed as a reflector,an increase in temperature over the whole soot body surface is aimed atby predetermining the reflectance for the IR radiation. This has theconsequence that local temperature peaks are evened out, namelyindependently of whether said temperature peaks are created due to theburner movement, due to misalignments or differences between theindividual deposition burners, or due to leakage radiation.

When the planar element is designed as a heat sink, local temperatureincreases due to leakage radiation are prevented or reduced in that theleakage radiation is absorbed or dissipated. This procedure has also theconsequence that local temperature peaks are avoided.

To enable the planar element to develop one of said effects, it isdesigned either as a mirror element (reflector) which homogeneouslyreflects IR radiation and has a temperature-raising effect on the whole,or as a cooling body (heat sink) which homogeneously absorbs IRradiation and has a temperature-shielding effect. In the first-mentionedcase the surface design of the planar element is of essentialimportance, whereas in the second case the material of the planarelement also has some influence on the cooling function.

The planar element extends over a substantial part of the length of theforming soot body, its temperature-homogenizing function being all thebetter fulfilled the longer the length section of the soot body is thatis covered by the planar element. Since a planar element which isslightly shorter than the soot body may still show a homogenizingfunction to an adequate degree, a partial length of more than 50% of thesoot body length is still defined as a “substantial part” of said lengthfor reasons of clarity.

Of essential importance is the adjustment of the reflectance of theplanar element with the aim to even out the curve of the surfacetemperature and thus to homogenize the axial density profile of the sootbody. This adjustment of the effect of the planar element by surface ormaterial properties is made once at the beginning of a depositionprocess and is normally also maintained in the subsequent depositionprocesses.

The temperature adjustment body consists of a single planar element orit is composed of several planar elements. It is also possible toprovide a plurality of planar elements which differ from one another intheir homogenization effect with respect to intensity or with respect tothe type (as a homogeneous heat sink or as a homogeneous reflector), butit is always ensured that one of the planar elements extends along asubstantial part of the SiO₂ soot body.

Advantageous developments of the device according to the inventionbecome apparent from the subclaims. Insofar as designs of the device asindicated in the subclaims copy the procedures mentioned in subclaimswith respect to the method according to the invention, reference is madeto the above comments on the corresponding method claims for asupplementary explanation. The designs of the device according to theinvention as mentioned in the remaining subclaims shall now be explainedin more detail.

With a planar element that has a concave curvature, the IR radiation canbe focused onto the surface of the soot body and the homogenizing effectcan thereby be intensified. The planar element is e.g. designed as aconcave mirror with a longitudinal axis extending along the soot body,the mirror surface extending around the whole cylinder outer surface ofthe soot body or a part thereof.

In this configuration of the device, two variants have again turned outto be equally suited.

In the first variant, the concave curvature has a focal point which islocated in the area of the row of the deposition burners. With theplanar element, the heat of the deposition burner is particularlyreflected towards the soot body. The planar element is arranged anddesigned such that heat emanating from the deposition burners arrangedin a row will impinge thereon and said heat will be reflected towardsthe forming SiO₂ soot body. The planar element may here e.g. be arrangedsuch that the row of the deposition burners or the rows of thedeposition burners extend between the soot body and the planar element.The lost heat radiated from the deposition burners to the rear is thusintercepted by the planar element and directed towards the forming sootbody.

In the second variant of the device, the concave curvature has a focalpoint which is located in the area of the forming SiO₂ soot body.

Heat emanating from the soot body is here intercepted by the planarelement and reflected back again towards the soot body surface. Theplanar element extends here preferably above, next to or below the sootbody.

A planar element acting as a heat sink is advantageously provided with acooling device.

The cooling device consists e.g. of a cooling body connected to theplanar element or of a flow means by which the planar element can beacted upon with a gaseous or liquid cooling medium. Thanks to thecooling of the planar element its efficiency can be varied withincertain limits for influencing and homogenizing the surface temperatureof the soot body.

The present invention will now be explained in more detail withreference to embodiments and a drawing, which schematically shows indetail in

FIG. 1 a longitudinal section through a first embodiment of the deviceaccording to the invention with two concave mirrors arranged laterallyrelative to the soot body, in a front view;

FIG. 2 the device according to FIG. 1 in a section taken along A-A′, ina side view; and

FIG. 3 a second embodiment of the device according to the invention witha cylindrical deposition chamber acting as a concave mirror, in a sidevide.

In the device which is schematically shown in FIG. 1, a carrier 1 ofaluminum oxide is provided inside a deposition chamber 8, the carrierbeing rotatable about its longitudinal axis 3 and a porous soot body 2of SiO₂ particles being produced thereon by means of deposition burners5. The deposition burners 5 are mounted in a row parallel to thelongitudinal axis 3 of the carrier 2 on a joint burner block 4. The SiO₂particles are deposited by reciprocating the burner block 4 at anamplitude of 20 cm (block arrow 6). The burner block 4 is connected to adrive which effects its reciprocating movement. Each of the depositionburners 5 are fed with burnable gases, oxygen and hydrogen and withvaporous SiCl₄ as the starting material for forming the SiO₂ particles.The distance between the surface 10 of the soot body 2 and the burnerblock 4 is kept constant in the deposition process. To this end theburner block 4 is movable in a direction perpendicular to thelongitudinal axis 3 of the carrier 1, as outlined with directional arrow11.

With the deposition burners 5, SiO₂ particles are deposited on thesurface 10 of the soot body 2 which is rotating about the longitudinalaxis 3 of the carrier. The deposition burners 5 are here reciprocatedalong the soot body surface 10 at identical movement cycles betweenlocally constant turnaround points. The peripheral velocity of the sootbody 2 is kept constant at 10 m/min in the deposition process. The meantranslational velocity of the burner block 4 is 350 mm/min.

Moreover, the device is equipped with homogeneous planar elements actingas reflectors in the form of two concave mirrors 13 which are oppositeeach other on the soot body 2 and extend at both sides of the soot body2 over the whole length thereof. The concave mirror 13 consists ofspecial steel, and the concave inner curvature facing the soot body 2 iseach time mirror-finished, whereby its reflectance for infraredradiation is approximately 100%. The concave mirror 13 has a radius ofcurvature of 400 mm and the distance to the longitudinal axis 3 of thecarrier is 270 mm. The focus line 14 (see FIG. 2) of the two concavemirrors 13 extends each time in parallel with the longitudinal axis 3 inthe area of the surface 10 of the soot body 2. To keep the focus line 14with an increasing outer diameter of the soot body 2 in said area, theconcave mirror 13 is movable in a direction perpendicular to thelongitudinal axis 3 of the carrier, as outlined by the block arrow 17.The efficiency of the two concave mirrors 13, defined as the solid anglecovering the forming SiO₂ soot body, is about 80%.

FIG. 2 shows the device according to FIG. 1 in a side view. As can beseen, the concave mirror 13 has an inner curvature which imitates thespatial shape of the forming soot body 2. The concave mirrors 13 extendat both sides of and in parallel with the burner row 4, the minimaldistance between the concave mirrors 13 and the soot body surface 10being kept constant at a value of 100 mm in that the concave mirrors 13are moved in the direction of the block arrow 17 in the build-upprocess. The focus line 14 of the concave mirror 13 extends each time ina direction perpendicular to the sheet plane along the soot body surface10.

The concave mirrors 13 reflect lost heat emanating from the soot body 2back onto the soot body surface 10, namely over the whole length of thesoot body 2. This contributes to a heating of the soot body, wherebyvariations in the surface temperatures are evened out. It is thuspossible to produce a soot body 2 with an axially homogeneous densityprofile. It has been found that the use of the concave mirrors 13increases the density of the soot body 2 by 1.5% on average. Theincrease in density can be compensated by reducing the burnable gasessupplied to the deposition burners 5, a reduction of the burnable gasesO₂ and H₂ by 5% being required in the embodiment.

In a first alternative embodiment of the device according to theinvention the concave mirrors which are opposite each other on the sootbody only extend over about 80% of the soot body length.

In a second alternative embodiment the concave mirrors which areopposite each other on the soot body also extend over about 80% of thesoot body length and are each extended at both sides beyond the sootbody ends by means of special steel elements that have a matsand-blasted surface. The matted surfaces act in the area of the twosoot body ends as a heat sink which leads to a reduction of the densityin said areas, as compared with the above-explained first alternativeembodiment.

Insofar as like reference numerals as in FIGS. 1 and 2 are used in theembodiment of the device of the invention as shown in FIG. 3, theserefer to identical or equivalent components of the device as in FIGS. 1and 2. Reference is made to the corresponding explanations.

In the device according to FIG. 3, the deposition chamber 30 isdesignated as an elongate cylindrical concave mirror 31 with anelliptical cross-section which extends along the soot body 2 over thewhole length thereof. The concave mirror 31 consists of special steel,and the concave inner curvature 33 facing the soot body 2 is heremirror-finished and has a reflectance for infrared radiation ofapproximately 100%. An exhaust gap 36 extends at the upper side of theconcave mirror 31, and at the lower side thereof an elongate penetration37 is provided for longitudinally guiding the burner block 4 and forsupplying the burnable gases.

The focus lines 34, 35 of the concave mirror 31 extend (in a directionperpendicular to the sheet plane) in parallel with the longitudinal axis3 of the carrier. The soot body surface 10 is held in the one focus line34 of the concave mirror 31 (focal point) in that the carrier 1 with anincreasing outer diameter of the soot body 2 is shifted upwards in thedirection of arrow 38. The burner flames 18 of the deposition burners 5are positioned on the other focus line 35.

The concave mirror 31 reflects lost heat emanating from the burnerflames 18 back to the soot body surface 10, namely over the whole lengthof the soot body 2. This contributes to a homogeneous heating of thesoot body 2, so that the temperature of the deposition burners 5 islowered accordingly, and the inhomogeneous amount of the heat radiationrequired for soot formation is thus reduced in favor of an axially morehomogeneous heating. Variations in the surface temperature are thusevened out. As a result, it is possible to produce a soot body 2 with anaxially homogeneous density profile.

In a constructionally simple variant, the deposition chamber 30 ishowever configured as an elongate concave mirror with a circularcross-section, as explained with reference to FIG. 3. In thisembodiment, the focus line of the concave mirror (the central axis)extends in a direction perpendicular to the sheet plane and in parallelwith the longitudinal axis of the carrier advantageously between theburner flames and the soot body surface. The radius of curvature of theconcave mirror is 600 mm and its distance to the longitudinal axis ofthe carrier is 400 mm. The concave mirror designed in this way reflectslost heat emanating from the burner flames back onto the soot bodysurface, namely over the whole length of the soot body. In comparisonwith the embodiment of the invention as shown in FIG. 3, this leads,however, to a slightly lower efficiency with respect to the reflectionof the heat of the deposition burners onto the soot body surface.

For the explanation of a further variant of the device of the invention,reference is now made to the configuration shown in FIGS. 1 and 2. Aplanar element is here provided in the form of an upwardly open quartershell of polished special steel with a reflectance of almost 100%, whichshell extends below the whole burner block 4 and by means of which thelost heat of the deposition burners 5 which is emitted downwards isreflected back towards the soot body 2. The quarter shell is firmlyconnected to the burner block 4 and is reciprocated therewith along thesoot body 2, and with an increasing diameter of the soot body 2 it isshifted downwards with the burner block 4 to keep constant the distancebetween the burner flame and the soot body surface 10.

1. A method for producing a quartz glass blank, said method comprising:a method step in which SiO₂ particles are produced by a row ofdeposition burners and deposited on a cylinder outer surface of acarrier rotating about a longitudinal axis thereof to form a cylindricalporous SiO₂ soot body, a temperature adjustment body altering a surfacetemperature of the soot body as it is being formed, wherein thetemperature adjustment body comprises a planar element extending along asubstantial part of the SiO₂ soot body, which either acts as ahomogeneous heat sink and has a temperature-shielding effect on the sootbody surface or, acts as a homogeneous reflector, and has atemperature-raising effect due to heat radiation.
 2. The methodaccording to claim 1, wherein said planar element is formed by an innerwall of a housing surrounding the SiO₂ soot body.
 3. The methodaccording to claim 1, wherein the planar element acts as a reflectorwith a reflectance for IR radiation between 80% and 100%.
 4. The methodaccording to claim 3, wherein heat of the deposition burners isreflected towards the soot body by means of the planar element.
 5. Themethod according to claim 3, wherein heat of the forming SiO₂ soot bodyis reflected by means of the planar element towards the soot bodysurface.
 6. The method according to claim 1, wherein the planar elementhas an efficiency, defined as a solid angle covering the forming SiO₂soot body, of at least 60%.
 7. The method according to claim 1, whereinthe planar element acts as a heat sink absorbing IR radiation.
 8. Themethod according to claim 7, wherein the planar element has a roughenedsurface having a mean surface roughness R_(a) of at least 10 μm.
 9. Themethod according to claim 7, wherein the planar element has a blackenedsurface.
 10. The method according to claim 7, wherein the planar elementis cooled.
 11. The method according to claim 3, wherein the planarelement is moved along the soot body.
 12. The method according to claim3, wherein the distance between the planar element and the surface ofthe forming SiO₂ soot body is kept constant.
 13. The method according toclaim 1, wherein the planar element extends over the whole usable lengthof the soot body.
 14. A device for carrying out the method according toclaim 1, said device comprising: a row of deposition burners forproducing SiO₂ particles, a carrier which is rotatable about thelongitudinal axis thereof and having a cylinder outer surface on whichthe produced SiO₂ particles are deposited to form a cylindrical porousSiO₂ soot body, and at least one temperature adjustment body that issupported in an area of the forming soot body and that acts on a surfacetemperature of the forming soot body for altering an axial densityprofile, wherein the temperature adjustment body comprises a planarelement that acts as a homogeneous heat sink or as a homogeneousreflector and that extends along a substantial part of the SiO₂ sootbody and has a predetermined reflectance for IR radiation.
 15. Thedevice according to claim 14, wherein the planar element is formed by aninner wall of a housing surrounding the SiO₂ soot body.
 16. The deviceaccording to claim 14, wherein the planar element has a reflectancebetween 80% and 100% for IR radiation.
 17. The device according to claim16, wherein the planar element has a concave curvature.
 18. The deviceaccording to claim 16, wherein the concave curvature has a focal pointwhich is located in an area of the row of deposition burners.
 19. Thedevice according to claim 16, wherein the concave curvature comprises afocal point which is located in the area of the forming SiO₂ soot body.20. The device according to claim 14, wherein the planar elementcomprises a surface absorbing IR radiation.
 21. The device according toclaim 20, wherein the planar element is roughened and has a mean surfaceroughness R_(a) of at least 10 μm.
 22. The device according to claim 20,wherein the planar element has a blackened surface.
 23. The deviceaccording to claim 20, wherein the planar element is provided with acooling device.
 24. The device according to claim 16, wherein the planarelement is supported for movement along the soot body.
 25. The deviceaccording to claim 16, wherein the planar element is made displaceablein a direction perpendicular to the longitudinal axis of the carrier.26. The device according to claim 14, wherein the planar element extendsover the whole usable length of the soot body.